We know that at least some species of birds retain the capacity to replace hair cells which are lost following trauma or during aging. In quail, the ganglion cells of the VIIIth nerve, however, are reduced in number following hair cell loss, even though those hair cells are replaced. These results suggest that some active process must be involved if these new hair cells are to be innervated and influence auditory function. Currently little is known about the consequences of hair cell regeneration on hering. This proposal compares the functional consequences (i.e. the effects on hearing) of hair cell regeneration following acoustic insult in three different species of birds. The battery of behavioral tests consists of 10 psychoacoustic measures including new, comprehensive measures of the shape of the auditory filter and the shape of the temporal window. These have never been measured before in birds. Psychoacoustic measures will be conducted in parallel with anatomical investigations on the same animals and animals exposed under identical conditions to assess changes occurring to hair cells, innervation patterns, stereocilia, tectorial membrane, and tegmentum vasculosum. Pilot data on quail have shown that the time course of hair cell regeneration at the basal end of the papilla coincides well with the time course of recovery from TTS. The comparative approach to this problem should provide insight into the anatomical changes important for recovery versus those that are not. Other behavioral experiments on the precision of vocal production and vocal learning will assess how well anatomical and psychoacoustic recovery is in supporting other natural, auditory-dependent behaviors important for communication. These experiments will provide the most comprehensive data base we know of on the functional consequences of replacing the peripheral sensory epithelium in a mature animal. These results will have relevance for the direction of future studies on regeneration of the nerve and sensory cells in the cochlea and their relevance for hearing. We expect these findings to also have relevance for any future efforts to replace the sensory epithelium in mammalian and, hopefully, the human cochlea.